Re: [HTCondor-users] Segfault by the ClassAd python-wrapper

[Jason Patton <jpatton@xxxxxxxxxxx>]

Mingxuan,

Thank you for your detailed report! We have created a ticket for this issue:

https://htcondor-wiki.cs.wisc.edu/index.cgi/tktview?tn=6721

Jason Patton

On Fri, Jul 27, 2018, 11:47 AM Lin, Mingxuan <mingxuan.lin@xxxxxxxxxxxxxx> wrote:

Package: htcondor-python-binding

Version: 8.7.8

Severity: normal

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Hi,

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I encountered a bug with the python-bindings of ClassAd objects. Below are the minimal codes to reproduce the problem:

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 Python 3.6.5 | packaged by conda-forge | (default, Apr 6 2018, 13:39:56)

ÂÂÂÂÂÂ [GCC 4.8.2 20140120 (Red Hat 4.8.2-15)] on linux

ÂÂÂÂÂÂ >>> import htcondor, classad

ÂÂÂÂÂÂ >>> coll Â= htcondor.Collector()

ÂÂÂÂÂÂ >>> schedd= htcondor.Schedd(coll.locate(htcondor.DaemonTypes.Schedd))

ÂÂÂÂÂÂ >>> print([(k,v) for k,v in schedd.query()[0].items()])

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ÂÂÂÂÂÂ Segmentation fault (core dumped)

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The root of this SEGFAULT is that the `items` method of `ClassAdWrapper` returns invalid references to its child objects. A ClassAd object usually contains some objects of nontrivial type, such as `classad.ExprTree`, which are destroyed when their parent is garbage-collected by the python runtime. Therefore trying to access such objects results in memory access violation (SEGFAULT). Here is the code section responsible for it: https://github.com/htcondor/htcondor/blob/3c73f7c8bcb9d4b99793cd3bc97251639e18b99d/src/python-bindings/classad.cpp#L993

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This is a python3-only problem because the `items` method of python 3 object returns an `iterator` instead of a list of tuples.

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The proposed solution is to properly transform any non-trivial child object of `ClassAdWrapper` into a python object (with its own reference counter) so that the runtime can garbage collected them correctly.

Â

Cheers,

Mingxuan Lin

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